add support for absolute path instead of .ftppass/env variables

To follow-up on my previous pull request (#47), this commit adds support for passing the absolute path to a JSON file in the `authKey` parameter instead of the _.ftppass_ config or an environment variable.

The user can seamlessly specify either of these, the grunt task will first check for an environment variable, then a path, or finally default back to _.ftppass_

**Example:**

_/home/JohnDoe/sftp/my-json-file.json_
```json
{
    "username": "your-username",
    "keyLocation": "/path/to/your/key",
    "password": "your-strong-password"
}
```

_Gruntfile.js_
```javascript
"sftp-deploy": {
    build: {
       auth: {
           host: 'my-ftp.com',
           port: '5678',
           authKey: '/home/JohnDoe/sftp/my-json-file.json'
       }
      ...
    }
}
```
`inKey !== null` could be caught earlier in the `else if` expression. I think if it's missing we'll probably get an error above anyway.